Process for reducing ore



United States Patent 3,206,300 PROCESS FOR REDUCING ORE Murray C. Udy,Niagara Falis, N.Y., Nicholas J. Themelis, Montreal, Quebec, Canada, andLeonard E. (lids, Grand Island, N.Y., assignors, by mesne assignments,to Independence Foundation, Philadelphia, Pa, 21 corporation ofDelaware, and Koppers Company, Inc., Pittsburgh, Pa., a corporation ofDelaware No Drawing. Filed Dec. 31, 1962, Ser, No. 248,219 2 Claims.(Cl. 75-11) This invention relates to metallurgy, in general, and moreparticularly to the treatment of ores and like materials in a rotatingkiln to effect dehydration and substantial prereduction thereof prior tosmelting. The invention is specifically concerned with improvedoperating techniques whereby the rate at which materials may be treatedwithin such kilns may be substantially increased or even doubled.

M-any advances have been mad-e in recent years in the design,construction and operation of rotating kilns, and particularly thoseemployed for the prereduction of ores. Starting from a simple rotating,refractory-lined inclined tube having a gas-fired burner at thedischarge end, advancements have included the injection of carbonaceousreducing material at intermediate points from the middle towards thedischarge end, injection of side air to burn carbon monoxide and othervolatiles issuing from the bed and the important discovery that with theuse of volatile coals and like materials a substantial amount ofhydrogen reduction will occur within the bed. A further advance inoperating techniques is that for optimum reduction when employing adeficiency of reductant the load factor of the kiln should not exceedabout 15%. Several of the foregoing advances are described in detail incopending application of Franklin C. Senior and Chester E. Shaffer,Serial Number 145,865, filed October 18, 1961.

In spite of the foregoing advances, it has been determined that undermost operating conditions reduction is generally confined to thatone-fourth of the kiln nearest to the discharge end. That is, theconditions of a substantially reducing atmosphere and a temperaturesufficient to let reduction proceed at a substantial rate are generallyconfined to this area. While it is, of course, necessary to devote aportion of the kilns length to the dehydration of the ore charge and tooxidation of such materials as sulfur and arsenic, it was believed thatvery material improvements in kiln operation would result if theeffective reducing zone within the kiln could be enlarged. Of course,this had to be done in a manner which did not involve safety hazards andwhich was economic in practice. That these ends have been accomplishedby means of the present invention is obvious from the detailed exampledata set forth hereinbelow.

As is well known, the reduction of iron oxides in the solid state willtake place from a temperature of about 500 C. up to the point of fusion,but subject to varia tion depending on composition and so forth. It isalso known from thermodynamic and kinetic studies that reduction byhydrogen is favored at the lower temperature in this range whilecarbonaceous reduction is most efficient at the higher temperatures.With these considerations in mind and with the end result of increasingthe length of the reduction zone in view, it was found that the resultdid come about when the kiln was operated by injecting an excess ofnatural gas or other reducing gas at the discharge end of the kiln andthe kiln was operated at a slight positive pressure to keep outside airfrom coming in; also, high volatile coal was added at or near the feedend of the kiln, and side air was injected quite near the feed end so asto burn the excess gas and raise the temperature at this point muchhigher than had heretofore been possible. Moreover, by raising the bedtemperature as early as possible in its traverse of the kiln length andby providing the high-volatile coal at or before this point was reached,it was found that by the time the bed reached the temperature requiredfor reduction to proceed, it was substantially permeated with hydrogenand hydrocarbons resulting from volatilization of the coal. The hydrogenwas thus present at the temperature range where its efficiency washighest. The coal was effectively coked and the carbon reductionproceeded at the higher temperatures as the bed moved towards thedischarge end. Under these operating conditions, it was found that thethroughput of the kiln could be effectively doubled while stillmaintaining the desired prereduction of approximately Retention time wassubstantially reduced, and it was not necessary to exceed the desiredmaximum load factor of 15%.

The following detailed discussion of modified kiln oper-ation wascarried out on an eighty-foot kiln having an inside diameter ofapproximately four and one-half feet. The kiln had three coal feedersand five side air injectors,

but none within 35 feet of the feed end. Firstly, the seal between thedischarge end and its housing was renewed and tightened. It was found,however, that the kiln was somewhat out of round, and even with the sealtightened there was not direct contact of all points. By running thekiln under pressure it was possible to insure that gas would leak outrather than air leak in.

Piping for excess fuel gas was installed in the discharge-end housing atthe floor line, about 2- /2 feet below v the bottom of the kiln. Gas wasadmitted at two points. Open-end %-ll'lh pipes were used to bleed in thegas. Two extra side-air ports with the necessary fans were installed,one nearer to the discharge end and the other near the feed end. Thelatter was of larger size than the others. An air-operated hydrauliccylinder was installed on the discharge gate of the sinter-receivinghopper to allow quick opening and closing and reduce gas leakage out orair leakage in. Finally, it was necessary to install a water spray onthe blades of the flue-gas exhaust fan, since it was realized that theflue gas would be much hotter than before.

The kiln was heated slowly to a dull red heat at the discharge end usingthe (old) primary gas burner only. After a day, the primary gas wasincreased, and as soon as the kiln was red at the first air port, about15 feet from the end, auxiliary gas was turned on and the air portopened. This gave a flame of air burning in gas which heated the kilnfurther upstream. The process was repeated at succeeding air ports,using more and more auxiliary gas, until the kiln was bright red alongalmost its entire length. It required about 14 hours for this preheatingstep.

Due to safety considerations, more air then necessary for completecombustion was admitted at the air ports, and the oxygen in flue gasduring the period was 0.9 to 2.0% with no carbon monoxide, no hydnogenand methane. The flue-gas temperature out of the kiln was about 900 F.at the end of the period.

Iron ore at 2000 pounds per hour was started, and when the ore reachedthe side-coal feeders, high-volatile coal was fed in at each feeder.Side air was increased and fuel gas reduced to provide air to burn thevolatile matter of the incoming coal. Two hours after start of feeding,the ore feed was increased to 4000 pounds per hour, and this rate wasmaintained throughout most of the run.

A small amount of coal was fed with the ore throughout the run, in orderto provide reductants in the feed of the kiln (the first coal feederdownstream is 35 feet from the feed end). The quantities of materialsfed during the entire runare listed in Table I. Ore and other materials3 were fed continuously for 96 /2 hours with the exception of 1% hourson the first day when trouble was had with the kiln ore feeder. Thefeeding time therefore was 95 During the first hours of the run, at 4000pounds per hour ore feed rate, no metallic iron was found in the sinter,indicating something less than 33% reduction. After cutting the rateslightly metallics began to appear in sinter in a small amount (up to3%). However, it was 26 hours before conditions in the kiln could be setto obtain good reduction. Then followed a period of 10 consecutive hourswherein the metallic iron ran from 7 to 13%, averaging 9.4%. This isequivalent to 44.3% reduction. For two hours during this period themetallic iron averaged 13.6%, equivalent to 50.0% reduction. A summaryof the reduction results is shown in Table II.

T ABLE II.REDUCT IO'N The particular ore treated during the test wasselected because previous tests thereon indicated it was very difficultto prereduce. The excellent results obtained testify to the efficacy ofthe modified practice of the invention. It is to be noted that duringthe run, with efforts being concentrated to increase the feed-endtemperature, the temperature at the discharge end was neglected, andaveraged only about 1700 F., or 150 to 200 lower than the mid-pointtemperature. It is known that the discharge temperature should be ashigh as is practical, to obtain high reductions, but, of course, longerretention time at a lower temperature produces an equivalent effect andreduces the change for sintering.

On three occasions during the run, doughnut-shaped rings built up on thewalls some distance upstream from the feed end, usually near one or bothof the coal feeders. In all cases the rings fell from the walls aftersome hours without any action by the operators. They were softly fusedcakes.

The formation of rings indicate that the kiln was operated at/ orslightly above the maxi-mum temperature for this ore at certain areas inthe kiln, but this can readily be adjusted to prevent ringing at anypoint, and yet .have a sufi'iciently hot kiln throughout.

It should be noted that when operating with an excess of gas,temperatures must be controlled with air input; in normal combustion inexcess air, temperature is controlled by gas input.

An average of 9.3% carbon was detected in the sinter during the entirerun, and 10.6% during the 10-hour higherreduction period. This is asufficient quantity for complete reduction in the smelting furnace.

The kiln was operated at 0.46 r.p.m. during the entire run, andretention time was about 2 hours, 55 minutes. The load factor (percentloading) varied between 7.5

and 8.9%. As noted hereinabove, load factor is important because a bedwhich is too deep is difiicult to heat through and to circulate reducinggases through, thereby decreasing reduction. As noted in theaforementioned copending application, load factor should be kept below15% and the optimum range is about 6l1%.

The importance of hydrogen reduction is attested to by the fact thatduring the 10-hour period during the run when reduction was highest,hydrogen reduction within the bed was equivalent to 82 pounds of carbon.

The economic benefits accruing through use of the process of theinvention are attested to by the fact that, in spite of using as much as10 times more reducing gas than had previously been used, the increasein production resulted in a saving of $3.20 per ton of ore treated, whencompared to the best previous results with the same ore being treated inthe same kiln.

It is believed that a better understanding of the invention will begained by referring to the appended examples which are intended to beillustrative only and not in a limiting sense.

Example I A 20 in. I. D. x 20 ft. long kiln was modified to allow thepractice of the invention. Two air fans were installed along the lengthof the kiln to allow controlled combustion of the excess gas. Thedischarge end was sealed to retard .air inleakage. The discharge drumwas also tightly sealed to the discharge hopper. The kiln was inclined2. 5" and was operated at /2 r.p.m. The temperature of the kiln bed inthe first four feet from the feed end increased from 600 C. to 900 C.;the rest of the kiln was maintained between 900 C. to 1050 C.

After heating up, the kiln was operated under stable conditions for 44hours using two different iron ores. Feed rate was 180 pounds of ore.This gave a kiln loading of about 10%. The reductant was added with oreat the feed end of the kiln in an amount to give 10% carbon in excess ofthat to completely reduce all of the iron to metallic iron.

Anthracite fines were used with both ores. A medium volatile bituminouscoal was also studied with one of the ores. Sinter discharge averaged165 pounds of sinter per hour.

The main burner supplied 3600 s.c.f.h. of air and 400 s.c.f.h. ofnatural gas. An auxiliary burner at the discharge end supplied anadditional 900 =s.c.f.h. of natural gas. Thus the air to gas ratio atthe discharge end was 2.8 to 1. During the run, the side fan nearest thedischarge end supplied 2,500 s.c.f.h. of air and the second fan anadditional 4,000 to 6,000 s.c.f.h. of air.

When the kiln reached stable conditions, prereduction of the first oreranged from 50.2% to 60.3%. Prereduction for the second ore usinganthracite was from 45 to 54% and using the low volatile bituminous from40.8% to 47.5%.

Example II A rn'agnetitehematite calcine was processed in an ft. kilnusing the kiln practice of the invention. The carbonaceous reductant wasa finely divided anthracite low in volatile content. For the run, thekiln was already hot having completed a previous phase of operation.

The k-iln was maintained under an excess of gas pres sure and the oreand coal were fed together through the feed end of the kiln. Since therewas no interruption in feeding between phases, the kiln was operated foreight hours while feeding the new mixture in order to completely drainout the materials from the first operation. During the next 40 hours ofoperation, the following materials were fed to the kiln:

54 tons of calcine 11.6 tons of anthracite 22,900 s.c.f. gas

Feed rate averaged 2,700 lbs. of calcine.

A total of 37.2 tons of sinter was discharged from the kiln at anaverage temperature of 1010 C. Prereduction of the ore ranged from 45.2%to 69.9% throughout the 40 hour production period. The averageprereduction for this period was 60.2%. Metallic iron in the sinter was26% during optimum operating conditions.

In comparison, using previous kiln techniques, it has not been possibleto exceed a feed rate of from about 1300 to 1500 lbs. of ore or calcineper hour. Even at this lower feed rate, prereductions seldom exceeded50%.

The heat balance calculated for the run indicated that even with theexcess gas in the kiln and the gas leakage at the nose ring, the kilnhad operated at 37.4% thermal efficiency. If the gas leakage wereeliminated as in the case of a new kiln, the thermal efficiency wouldhave been 42.3

It will be understood that various changes in the details, materials,steps and arrangements of parts, which have been herein described andillustrated in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims.

Having thus described the subject matter of our invention, what it isdesired to secure by Letters Patent is:

1. Process for maintaining reducing conditions over approximatelytwo-thirds of the length of a rotating kiln for use in the continuoustreatment of a metallurgical charge that comprises:

charging a mixture of ore and carbonaceous rediictant into a rotarykiln,

'sa-id mixture occupying no more than fifteen (15%) percent of thecross-sectional area of said kiln;

at least partially oxidizing said mixture at an elevated temperaturewithin the one-third of said kiln nearest the feed end to elfectdehydration thereof and the removal of volatile and oxidizablecomponents therefrom including sulfur and arsenic;

maintaining a higher than atmospheric pressure of reducing gas adjacentthe discharge end of said kiln;

injecting sufiicient air at a first air-injection point no more thanabout one-fourth the length of said kiln from said feed end to burn allreducing gas between said point and said feed end;

injecting a volatile carbonaceous material between the feed end of saidkiln and said first air-injection point;

injecting additional carbonaceous material and air at a plurality ofpoints between said first air-injection point and the discharge end ofsaid kiln, the quantity of said additional carbonaceous material beingsuffic-ient to establish the total residual carbon content of themixture as discharged from said kiln at no more than about a 10 percentexcess of the stoichimetric quantity required for effecting the degreeof total metallization desired upon subsequent smelting of said kilndischarge, and

supplying said kiln discharge while hot to an electric smelting furnaceand smelting the same therein to totally reduce to the metallic statethe desired metallic values thereof.

2. The process as claimed in claim 1, wherein said charge mixtureoccupies between 6 and 11 percent of the cross-sectional area of saidkiln.

References Cited by the Examiner UNITED STATES PATENTS 1,760,078 5/30Newkirk 753 6 1,937,822 12/33 Jones 75-36 2,754,197 7/56 Wienert 75362,829,042 4/ 58 Moklebust 75-36 2,941,791 6/ 60 Wienert -36 3 029,141 4/62 Sibakin 75-34 DAVID L. RECK, Primary Examiner.

WINSTON A. DOUGLAS, Examiner.

1. PROCESS FOR MAINTAINING REDUCING CONDITIONS OVER APPROXIMATELYTWO-THIRDS OF THE LENGTH OF A ROTATING KILN FOR USE IN THE CONTINUOUSTREATMENT OF A METALLURGICAL CHARGE THAT COMPRISES: CHARGING A MIXTUREOF ORE AND CARBONACEOUS REDUCTANT INTO A ROTARY KILN, SAID MIXTUREOCCUPYING NO MORE THAN FIFTEEN (15%) PERCENT OF THE CROSS-SECTIONAL AREAOF SAID KILN; AT LEAST PARTIALLY OXIDIZIG SAID MIXTURE AT AN ELEVATEDTEMPERATURE WITHIN THE ONE-THIRD OF SAID KILN NEAREST THE FEED END TOEFFECT DEHYDRATION THEREOF AND THE REMOVAL OF VOLATILE AND OXIDIZABLECOMPONENTS THEREFROM INCLUDING SULFUR AND ARSENIC; MAINTAINING A HIGHERTHAN ATMOSPHERIC PRESSURE OF REDUCING GAS ADJACENT THE DISCHARGE END OFSAID KILN; INJECTING SUFFICIENT AIR AT A FIRST AIR-INJECTION POINT NOMORE THAN ABOUT ONE-FOURTH THE LENGTH OF SAID KILN FROM SAID FEED END TOBURN ALL REDUCING GAS BETWEEN SAID POINT AND SAID FEED END; INJECTING AVOLATILE CARBONACEOUS MATERIAL BETWEEN THE FEED END OF SAID KILN ANDFIRST AIR-INJECTION POINT; INJECTING ADDITIONAL CARBONACEOUS MATERIALAND AIR AT A PLURALITY OF POINTS BETWEEN SAID FIRST AIR-INJECTION POINTAND THE DISCHARGE END OF SAID KILN, THE QUANTITY OF SAID ADDITIONALCARBONACEOUS MATERIAL BEING SUFFICIENT TO ESTABLISH THE TOTAL RESIDUALCARBON CONTENT OF THE MIXTURE AS DISCHARGED FROM SAID KILN AT NO MORETHAN ABOUT A 10 PERCENT EXCESS OF THE STOICHIMETRIC QUANTITY REQUIREDFOR EFFECTING THE DEGREE OF TOTAL METALLIZATION DESIRED UPON SUBSEQUENTSMELTING OF SAID KILN DISCHARGE, AND SUPPLYING SAID KILN DISCHARGE WHILEHOT TO AN ELECTRIC SMELTING FURNACE AND SMELTING THE SAME THEREIN TOTOTALLY REDUCE TO THE METALLIC STATE THE DESIRED METALLIC VALUESTHEREOF.